To date a variety of beverages using acidified milk using skim milk as the starting material have been studied and developed. Of these products, very few have been offered as the ready-to-drink type. On the other hand, products of acidified thick milk beverage with much sugar which are diluted with water or carbonated water prior to drink have been consumed widely. Owing to excessively high sugar contents, however, these products suffers from the disadvantage that they must be diluted each time they are consumed. Furthermore, the solutions prepared by diluting said products are intended to be immediately consumed and, therefore, have another disadvantage that they readily produce a precipitate wheh they are left to stand for a long period. Even if the conventional acidified thick milk beverages are simply diluted, the resultant diluted solutions are unmarketable as beverages of the ready-to-drink type because of the interval between the time they are prepared and the time they are consumed by individual users. This is because the products in their thick state have so high sugar contents that high viscosities of the products prevent milk protein particles from precipitating but, once diluted, they can no longer be expected to provide said effects and specific gravity of sugar content. Thus, the milk protein particles settle down to form a precipitate because of the force of gravity.
In an effort to develop beverages of the ready-to-drink type, various attempts have so far been made at devising proper means for addition of acids, elucidating the relation between the concentrations of acids and the milk solids-not-fat contents, incorporating proper use of proteases and ensuring the stability of beverage constitution by heating. Commercial-scale production of beverages which possess tasty flavor and, at the same time, provide thorough stability of milk protein dispersion is so difficult that very few beverages have had favorable growth as really marketable products. For example, the stabilization of beverage constituent by heating has been disclosed in U.S. Pat. No. 3,800,052 owned by Calpis Shokuhin Kogyo Kabushiki Kaisha. As concretely disclosed in experiments and examples of said prior art, the heating is carried out at temperatures not exceeding 100.degree. C. U.S. Pat. No. 3,800,052, furthermore, has a restriction that sugar must be added prior to the incorporation of an acid to produce a thoroughly stable beverage.
In addition to those methods enumerated above, methods resorting to addition of thickeners and stabilizers have been studied. Having ascertained that such additives impair the beverage flavor and, when left to stand in the acidified condition for a long period, tend to degrade themselves and further that use of these additives frequently pose a serious issue relating to spoilage of natural food and hygienic valuation of food, the inventors have avoided using these additives in the development of products.
As already stated, the inventors have carefully reviewed the conventional techniques and have made an intense study in search of a method simple to perform and yet highly effective. The inventors have consequently succeeded in improving the art of U.S. Pat. No. 3,800,052. To be specific, the inventors have found that a heat treatment given at temperatures exceeding 100.degree. C. to an acidified milk under specific conditions described afterward without having to add any sugar prior to incorporation of an acid is notably effective in stabilizing the dispersion of milk protein and that said treatment yields a product of good taste.